Move actions source files to src/common/actions directory

Since more actions will be added, group them under an "actions"
directory.  The files to be added are expected to have some pretty
generic/overloaded names, such as start-session.c and snapshot.c, so
having them under the actions directory make it clear that they
implement the action described by the name.

Change-Id: Ia47160dd75531eb9bcf13f875f4bc6caa2391d7b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>